The present invention relates to the measurement of wood, especially by means of optical radiation.
In mechanical and chemical forestry, one of the most important wood handling processes is the barking of logs. In the production of energy as well, the handling of logs is usually begun by barking and the amount of bark has an impact on the burning process in terms of its regulation and efficiency. In the production of pulp, the barked trees are first chipped, whereafter the chips proceed to a pulp digester. The bark remaining on chips deteriorates the quality of pulp being produced and causes a need of adjustment in the digestion process. Mechanical pulp is produced by grinding or refining. In grinding, the barked log is pressed against a grindstone. In refining, the barked logs are first chipped and the chips are refined between two rotary wheels. However, it is not worth while to bark the logs too thoroughly, as a result of this is the removal of pure wood material along with the bark, leading to losses of energy and material. It has been a common attempt to regulate the barking process in view of optimizing the amount of bark in wood chips. The amount of bark typically accepted in a pulp mill is less than 0.5%-1%, and in a paper mill even less than 0.1% of the total mass of refined or ground mechanical pulp. The adjustment of a barking process requires information about the purity grade or thoroughness of barking.
In prior art solutions, the purity grade of barking is measured by imaging the logs or chips to be barked and by applying various computer-based image processing programs for assessing the respective proportions of wood and bark. Indeed, there is such a distinction between bark and pure wood body that bark is usually darker than pure wood body. A problem in this type of method is that it is difficult to distinguish the dark bark for example from shadows. Moisture causes reflections, impeding the detection of bark material present in the chips or body. In addition, when measuring tree trunks, it is difficult to make a distinction between the pure wood material and the bark as a result of the geometrical patterns of a wood surface. The problem is particularly pronounced when the amount of bark is small.
In another prior art solution, the purity grade of barking has been measured by estimating the proportion of pure body material in barking refuse. However, the measurement does not correlate particularly well with the bark remaining affixed to a tree trunk, since the barking process makes it necessary to break dry wood more than moist wood. Thus, the barking refuse of dry wood contains a greater amount of pure body material than the barking refuse of moist wood, even though both logs would have an equal amount of bark affixed thereto.
The invention seeks to provide such a method, and such an arrangement for implementing the method, that the above problems can be solved.
It is an object of the invention to provide a method for measuring the quality of wood, in which method the wood is constituted by timber which, in addition to a pure body, includes at least a bark component and knot components, which differ from the pure body in terms of optical properties thereof, the method involving the barking of logs. Furthermore, the method of the invention comprises turning at least some of the barked trees into wood meal or powder; exposing the wood meal to optical radiation; and measuring the quality of wood by means of the optical radiation.
It is another object of the invention to provide a measuring arrangement for the quality of wood, wherein the wood refers to timber which, in addition to a pure body, includes at least a bark component and a knot component, having optical properties different from those of the pure body, said measuring arrangement comprising a stripper for the barking of logs. Furthermore, in the arrangement of the invention, the measuring arrangement is adapted to produce wood meal or powder from barked trees; the measuring arrangement comprises a detector responsive to optical radiation; the measuring arrangement comprises a measuring device; and the detector is adapted to receive optical radiation coming from the direction of wood meal or powder and to carry a signal responsive to the optical radiation to the measuring device, and the measuring device is adapted to measure the wood meal by means of an optical-radiation based signal coming from the detector, and to determine the quality of wood.
A number of benefits are gained by the method and system of the invention. The amount of bark, knots, and/or defective wood in timber can be reliably measured without being interfered by the shape, shadows, or moisture (moisture-caused reflections) of tree trunks. Furthermore, the barking process and for example the digestion of chemical pulp can be optimally controlled according to the quality of wood.